The recent progress in wider bandwidths and faster data rates in mobile radio communication services has led to the realization of multimedia services such as music and video. With an aim of realizing even faster data services, 3GPP, a standards body of mobile communication technology, is in the process of standardizing mobile communication technology referred to as LTE (Long Term Evolution) which is capable of realizing data rates in excess of 100 Mbps.
In LTE, when an idle radio terminal receives a paging message or when a radio terminal initiates data transmission, a random access procedure using a random access channel (RACH) is performed for the purpose of establishing a connection with a radio communication base station device (3GPP Specification TS36.321. v8.1.0, “Medium Access Control specification”).
As illustrated in FIG. 23, a random access procedure is made up of four messages: A random access preamble signal (hereinafter referred to as a “RACH preamble signal”, and also referred to as a Message 1 in the present specification) and a random access response signal (hereinafter referred to as a “RACH response signal”, and also referred to as a Message 2 in the present specification) are MAC (Media Access Control) layer signals. An RRC connection request signal (RRC Connection Request, also referred to as a Message 3 in the present specification) and an RRC connection setup signal (RRC Connection Setup, also referred to as a Message 4 in the present specification) are RRC (Radio Resource Control) layer signals.
Basically, a radio terminal selects a cell having a best reception signal level (best cell) and accesses the selected cell. Since a radio terminal remains in motion even when the random access procedure is in progress, a reception signal level of a cell attempting connection constantly varies. Therefore, a switchover to another cell may take place during a random access.
FIG. 24 is a diagram illustrating a sequence in the present LTE for performing a cell reselection during a random access procedure. Even after transmitting a RACH preamble signal to a radio base station device 100a of a first cell, a radio terminal device 110 continues detection of reception signal levels of the first cell and an adjacent second cell. In this case, a detection of a reception signal level is referred to as a cell reselection evaluation.
For example, there may be cases where an RRC connection setup signal message cannot be received and an RRC connection failure occurs despite transmitting a RACH preamble signal for a maximum number of transmissions or retransmitting an RRC connection request signal for a maximum number of retransmissions. In such a case, if the second cell is the best cell at this point, then cell reselection is performed to the second cell (3GPP TSG RAN WG2 Document, R2-081107 “Cell Reselection during RRC Connection Establishment”). In the example illustrated in FIG. 24, after the RRC connection failure, the radio terminal device 110 transmits a RACH preamble signal to a radio base station device 100b of the second cell.
In order to once again transmit a RACH preamble signal in the second cell that is the destination, the radio terminal device 110 must acquire parameters necessary for the RACH preamble signal. Parameters necessary for the RACH preamble signal are comprised in system information regularly announced by the radio base station device (eNodeB) 100b. System information is information for announcing parameters used by the respective cells.
Therefore, the radio terminal device 110 having performed a cell reselection must receive system information in order to acquire a preamble parameter used by the new cell. In LTE currently being specified, a preamble parameter is announced in an SIB2 (System Information Block 2) at a frequency of 160 ms (3GPP Specification TS36.331 v8.1.0, “Evolved Universal Terrestrial Radio Access (E-UTRA) Radio Resource Control (RRC) Protocol Specification”). This means that the radio terminal device 110 having performed a cell reselection must wait for a maximum of 160 ms before transmitting a RACH preamble signal. Such a delay may significantly increase the latency before a connection is established.
An evaluation of an IMT-Advanced system as a next-generation cellular communication system has been started. Compared to current systems, high frequency bands (3.4 to 3.6 GHz) are also allocated for the IMT-Advanced system. With the IMT-Advanced system, a cell radius may conceivably decrease due to the use of higher frequency bands. Accordingly, as illustrated in FIG. 25, the deployment of a radio relay station device 120 at a cell edge of the radio base station device 100 is being considered for IMT-Advanced for the purpose of improving cell edge performance and expanding cell coverage.
Since cell density is increased in a next-generation cellular communication network in which the radio relay station device 120 is deployed as described above, an increase in frequency of the aforementioned cell reselection during a random access procedure as well as an increase in latency upon connection establishment are predicted (IEEE P802.16j/DI (August 2007) “Part 16: Interface for Fixed and Mobile Broadband Wireless Access Systems”, IST-4-027756 WINNER II, D3.5.1 v1.0 “Relaying concepts and supporting actions in the context of CUs”).